The present invention relates generally to centrifugal compressors and, more specifically, in relation to certain embodiments, to a self-aspirated flow control system for a centrifugal compressor and a method for controlling flow in a centrifugal compressor using such a system.
Centrifugal compressors are often employed in various applications, such as chemical manufacturing, textile plants, petroleum refining, or the like. In fact, centrifugal compressors are employed to increase the pressure of a gaseous fluid, such as air for pumping, or for providing fluid to a downstream device such as a combustor or a turbine. One of the drawbacks arising in the use of centrifugal compressors for applications where the compression load varies over a wide range is flow destabilization (i.e., flow separation) through the compressor. Traditionally, compressor inlets, impellers, and diffuser passages are designed to accommodate a range of volumetric flow rates likely to be required. In the compressor, sometimes a minimum volumetric flow rate is encountered below which stable compressor operation may not be possible. Typically, high incidence on the impeller, boundary layer flow separation along the impeller length or boundary layer flow separation in the diffuser, de-swirl vanes, or interconnecting transition ducts initiates the unstable compressor operation referred as “stall”. Below the stall point, the pumping capability of the compressor is significantly reduced and large aerodynamic inefficiency results. Large stable operation range is a critical requirement for compressors to control the diffusion process that leads to boundary layer flow separation.
Traditionally, designers have increased the number of stages of compression or increased the length of interconnecting transition ducts of the compressor to reduce pressure gradients along the flow path and, thus, to prevent significant boundary layer growth which precedes boundary layer flow separation. Unfortunately, adding more compression stages or increasing the length of interconnecting transition ducts results in an increase in the overall dimensions of the compressor and an increase in the number of parts, both of which are generally undesirable.
In one example, traditional multi-stage centrifugal compressors and interconnecting transition ducts are usually compact to reduce the compressor dimensions such as length, diameter, and weight. But, if the interconnecting transition ducts are compact, larger pressure gradients are generated resulting in additional pressure losses. As a result, the efficiency of the compressor is reduced. In another example, diffusion of flow through the 180-degree bend portions of the interconnecting transition ducts between the compression stages is limited by increasing radius of the bend portions or by adding more turning vanes or other metal blockage. In turn, pressure losses are reduced in the interconnecting transition ducts. But increasing the radius of 180-degree bend portions of the interconnecting transition ducts or adding turning vanes or other metal blockage to reduce pressure gradients results in an increase in overall dimensions of the compressor, cost, and number of components.
Therefore, there is a need for an improved system and method for controlling flow in a centrifugal compressor.